Re: [PATCH v2 1/1] xattr: Allow user.* xattr on symlink and special files

From: Christian Brauner
Date: Fri Jul 09 2021 - 05:19:42 EST


On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote:
> Currently user.* xattr are not allowed on symlink and special files.
>
> man xattr and recent discussion suggested that primary reason for this
> restriction is how file permissions for symlinks and special files
> are little different from regular files and directories.
>
> For symlinks, they are world readable/writable and if user xattr were
> to be permitted, it will allow unpriviliged users to dump a huge amount
> of user.* xattrs on symlinks without any control.
>
> For special files, permissions typically control capability to read/write
> from devices (and not necessarily from filesystem). So if a user can
> write to device (/dev/null), does not necessarily mean it should be allowed
> to write large number of user.* xattrs on the filesystem device node is
> residing in.
>
> This patch proposes to relax the restrictions a bit and allow file owner
> or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs
> on symlink and special files.
>
> virtiofs daemon has a need to store user.* xatrrs on all the files
> (including symlinks and special files), and currently that fails. This
> patch should help.
>
> Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@xxxxxxxxxx/
> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> ---

Seems reasonable and useful.
Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>

One question, do all filesystem supporting xattrs deal with setting them
on symlinks/device files correctly?

> fs/xattr.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 5c8c5175b385..2f1855c8b620 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode,
> }
>
> /*
> - * In the user.* namespace, only regular files and directories can have
> - * extended attributes. For sticky directories, only the owner and
> - * privileged users can write attributes.
> + * In the user.* namespace, for symlinks and special files, only
> + * the owner and priviliged users can read/write attributes.
> + * For sticky directories, only the owner and privileged users can
> + * write attributes.
> */
> if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) {
> - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
> + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) &&
> + !inode_owner_or_capable(mnt_userns, inode))
> return (mask & MAY_WRITE) ? -EPERM : -ENODATA;
> if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) &&
> (mask & MAY_WRITE) &&
> --
> 2.25.4
>